Fix regression in buffered_write_stream.

asio/include/asio/buffered_write_stream.hpp

diff --git a/asio/include/asio/buffered_write_stream.hpp b/asio/include/asio/buffered_write_stream.hpp
index 4a04475..561b63f 100644
--- a/asio/include/asio/buffered_write_stream.hpp
+++ b/asio/include/asio/buffered_write_stream.hpp
@@ -334,7 +334,8 @@
     std::size_t bytes_avail = asio::buffer_size(buffers);
     std::size_t length = bytes_avail < space_avail ? bytes_avail : space_avail;
     storage_.resize(orig_size + length);
-    return asio::buffer_copy(storage_.data(), buffers, length);
+    return asio::buffer_copy(
+        storage_.data() + orig_size, buffers, length);
   }
 
   /// The next layer.